Drum brake systems for vehicles are well known. In such brake systems, the brake drum is used to generate braking torque by serving as a friction couple with brake shoe lining material. A brake drum is typically rigid and of a rounded or cylindrical geometry. It encases the actuating brake components, including the brake shoes, and is constructed of a cylindrical main body featuring a braking surface and a mounting or backing plate connected to the cylindrical main body by the drum wraparound. The drum braking surface is set in motion by the rotation of the vehicle wheel as it is attached to the wheel hub. The backing plate is bolted to the wheel and wheel hub and connects the braking surface of the drum to the rotating system through the wraparound.
When braking occurs, the brake shoes are actuated and move outward away from the center of the vehicle wheel rotation. As the shoes are actuated, they press against the inner diameter of the rotating drum's braking, surface converting kinetic energy to heat energy by means of friction.
As kinetic energy is converted into heat energy, it is desirable to remove as much heat from the system as possible so that more kinetic energy can be converted. One way of removing heat is by cutting vents or windows into the wraparound. Typically this design helps in dissipating heat, but, because the wraparound connects the braking surface to the fixed backing plate, it is subject to mechanical forces. The wraparound thus is a support structure and it is weakened by having material removed for forming the cooling vents or windows. The loading of the wraparound makes both wraparound design and vent or window design critical to the durability of the drum.
There are two common wraparound designs used in brake drums: (1) the straight wraparound, and (2) the shouldered wraparound. The straight wraparound extends from the backing plate at art angle to connect the cylindrical main body, The magnitude of this angle depends on the relation of the bolt-hole pattern diameter to the braking surface diameter. The two features are connected through a section of straight geometry. The shoulder wraparound serves to connect the same two features, but does so through two sections of straight geometries, both at different angles connected by a radius creating a “shoulder” shape. This design is not practical where the bolt-hole diameter is large with respect to the braking surface diameter.
A prior art brake drum is indicated in general at 10 in FIG. 1. The prior art brake drum features a cylindrical main body 12 and a backing plate 14 joined by a straight wraparound 16. As illustrated in FIG. 2, the braking surface 18 of the main body 12 is subjected to a force or load, indicated by arrow 22, by the brake shoe lining material (not shown) when the drum brake system is activated by a driver (such as by the driver pushing the vehicle brake pedal) to stop the vehicle.
As illustrated in FIGS. 1 and 2, the brake drum is provided with a number of cooling windows or vents 24 formed in the straight wraparound 16. As shown in FIG. 2, each cooling vent 24 is defined by a generally vertical outer edge 26 and a generally horizontal inner edge 28. When the load 22 is acting on the braking surface 18, a bending moment, illustrated in FIG. 2 by arrow 30. is formed by a torque arm 32 and a vector 34. This bending moment (30) acts on the inner edge 28 of the vent or window 24, In addition, a tensile force acts on the inner edge 28 of the vent 24, as indicated by vector 36 (FIG. 2). Due to the lengthy horizontal component of the torque arm 32, indicated at 38 in FIG. 2, the bending moment 30 acting on the inner edge of the vent is large as compared to the tensile force 36. The result increases the tendency of the formation of stress cracks on the inner edges of the cooling vents 24 of the prior art brake drum of FIGS. 1 and 2. Decreasing the bending moment 30, even if the tensile three 36 is increased as a result, would reduce the tendency for stress cracks to form on the inner edges of the vent or windows 24, and thus would increase the durability of the brake drum.
Window or vent designs are commonly square in shape with radii in the corners to reduce stress levels. It is common that they are cast-in features and must have a shape that is compatible with the casting process. They are located in the wraparound of the brake drum and have been manufactured in the straight type wraparounds. While it is desirable to provide additional material in the inner two corners of the windows or vents of a brake drum due to the bending moments and tensile forces acting on the inner edge of the window, such an addition of material must be balanced against providing a low brake drum weight and adequate cooling via the vents or widows.
A need exists for a brake drum that addresses at least some of the above issues.